1. Field of the Invention
The invention relates to the field of image data compression and decompression.
2. Background Information
Computer systems display both still and motion video images. Motion video usually consists of a sequence of images (referred to as frames) that, when displayed at a particular frame rate, will appear as "real-time" motion to a human eye. A still video image or a frame in a motion video sequence comprises a number of elements referred to as pixels (e.g., a 640.times.480 image comprises over 300,000 pixels). Each pixel is represented by a binary pattern that describes that pixel's characteristics (e.g., color, brightness, etc.). Given the number of pixels in a typical video image, storing and/or transmitting uncompressed still or motion video data requires a relatively large amount of computer storage space and/or bandwidth. Additionally, in several motion video applications, processing and displaying a sequence of frames must be performed fast enough to provide real-time motion (typically, between 15-30 frames per second). Techniques have been developed to compress the amount of data required to represent still and motion video, making it possible for more computing systems to process still and motion video.
One such compression technique is based on decomposing a frame into its different components using a digital filter, and then encoding each component using the coding technique best suited to that component's characteristics. To provide an example, subband coding in a technique by which each frame is decomposed into a number of frequency subbands, which are then encoded using the coding technique best suited to that subband's characteristics. As another example, various references describe different frame compression systems that are based on using wavelets to decompose a frame into its constituent components (e.g., U.S. Pat. No. 5,661,822; U.S. Pat. No. 5,600,373). For example, a frame compression system can decompose an image into a high frequency subband (H) and a low frequency subband (L), and then further decompose the H and L subbands to generate a high-high frequency part (HH), a high-low frequency part (HL), a low-high frequency part (LH), and a low-low frequency part (LL). Generally, the LL subband contains most of the signal energy, while the HH subband represents a relatively small percentage of the energy.
The results of the decomposition are typically quantized and encoded. While certain techniques have been described for quantizing and encoding the result of decomposed image data (U.S. Pat. No. 5,135,670; U.S. Pat. No. 5,412,741), lower processing complexity, higher quality, and/or higher compression ratio techniques are desired and/or required.